1. Field of the Invention
The present invention relates to organic semiconductor devices, field-effect transistors (FETs), and their manufacturing methods. In particular, the invention relates to a field-effect transistor including a gate insulating layer which can be easily formed from a solution and from which low-molecular-weight impurities or the like do not precipitate, and a method for manufacturing the same. The field-effect transistor can exhibit a high mobility even if the substrate is made of a resin.
2. Description of the Related Art
IC technology using organic semiconductor devices has recently received attention because they can be prepared at low cost and allow the use of a flexible resin substrate. Because of these advantages, the organic semiconductor devices promise to be applied to, for example, circuits using plastic boards, display drive circuits for electronic tags and displays, and memory.
An organic semiconductor device generally includes a substrate, insulating layers, electrodes, and an organic semiconductor layer. Among such organic semiconductor devices is a thin-film field-effect transistor including a gate insulating layer, a gate electrode, a source electrode, a drain electrode, and an organic semiconductor layer.
In a field-effect transistor using an organic semiconductor as the semiconductor layer, when the voltage (gate voltage, Vg) applied to the gate electrode is varied, the electric charge (carrier) at the interface between the gate insulating layer and the organic semiconductor layer increases or decreases. As a result, the value of drain-source current (Id, negative when flows from the drain electrode to the source electrode) flowing from the source electrode to the drain electrode through the organic semiconductor is varied. Thus, the field-effect transistor functions. The field-effect transistor functioning as the most ideal switching device can switch the states where carrier path is present and absent.
In fact, high-performance organic semiconductor devices are produced by applying a solution of an organic semiconductor compound, such as polyalkylthiophene or poly(thienylenevinylene) (see Assadi A. et al., “Field-effect mobility of poly(3-hexylthiophene)”, Appl. Phys. Lett., vol. 53, p. 195 (1988); Fuchigami H. et al., “Polythienylenevinylene thin-film transistor with high carrier mobility”, Appl. Phys. Lett., vol. 63, p. 1372 (1993); and Japanese Patent Laid-Open No. 10-190001).
In order to produce an inexpensive and flexible device, the electrodes and the insulating layers can be formed on a flexible resin substrate by coating or printing. However, the smoothness and flatness of the resin substrate is extremely inferior to those of silicon or glass substrates. Also, wet processes, such as printing, are easily affected by the surface state of the substrate. It is therefore difficult to form an insulating gate insulating layer with a uniform thickness and sufficient insulation properties on the resin substrate. Accordingly, the leak current from the source electrode to the drain electrode increases disadvantageously.
For the production of the organic semiconductor device on a flexible resin substrate, the components overlying the substrate, such as the gate insulating layer and the organic semiconductor layer, are preferably formed at low temperatures of 200° C. or less. The resin substrate may be softened and degraded in an atmosphere of high temperature.
Known insulating layers will be illustrate below.
For example, Simoda et al. have produced an field-effect transistor including an insulating layer formed of polyvinylphenol (PVP), and electrodes and an organic semiconductor layer formed by ink jet printing (Simoda T. et al., “Organic transistor fabricated by ink-jet printing” Oyo-buturi, Vol. 70, p. 1452 (2001). Veres et al. have produced a field-effect transistor including a polytriallylamine organic semiconductor layer on an insulating layer having a low relative dielectric constant (Veres B. J. et al., “Low-k insulator as the choice of dielectrics in organic field-effect transistors”, vol. 13, p. 199 (2003)). Both the studies above use thermoplastic resins for the insulating layers. The thermoplastic resin insulating layers exhibit high workability, but have problems with solvent resistance and thermal stability. Thus, the thermoplastic resin insulating layer is unsuitable for use in a multilayer structure, and it is difficult to form a highly insulating thin, dense layer.
Other approaches have been reported in which the insulating layer is formed of a thermosetting resin prepared by adding a methylated or acylated melamine-formaldehyde resin to PVP (Japanese Patent Laid-Open No. 2004-128469; Zschieschang, et al., “Flexible Organic Circuits with Printed Gate Electrodes”, Advanced Materials, vol. 15, p. 1147 (2003)). This type of insulating layers may allow unreacted polar groups to remain in the layer after baking the resin at low temperatures at 200° C. or less, and the insulation properties may be reduced due to hygroscopicity or the like. 
In addition, US2004094761 A1 and WO2002/009190 have disclosed improved insulating layers.
Each insulating layer above cannot ensure a high insulation or reliability if it is baked at a low temperature at which the plastic substrate can be maintained. If a catalyst or the like is added so that the insulating layer can be cured at a low temperature, the catalyst may undesirably contaminate the organic semiconductor layer. Thus, it has been difficult to achieve a field-effect transistor including an organic semiconductor layer with a high mobility and reliability.
As described above, it has been difficult to form a highly insulating uniform organic layer on a resin substrate in inexpensive manufacturing processes of field-effect transistors using organic semiconductors. Although thermally curable resin compositions prepared by adding a crosslinking agent to a thermoplastic resin, which is easy to apply for film formation, are useful to solve the difficulty, the thermally curable resin compositions cannot be sufficiently cured to form an insulating layer in the absence of a catalyst. Thus, it is difficult to form a highly insulating layer not adversely affecting the organic semiconductor layer. This is a challenge in the formation of not only the gate insulating layer, but also other insulating layers, and is also a challenge to field-effect transistors and other devices using organic semiconductors.